I live where the West begins, on the cactus-and-yucca-studded rim of a rock canyon through which flows a pristine stream fed by thousands of springs. I am not, as you might think, describing a remote ranch. I live less than a mile outside the city limits of Austin, Texas. The body of water is a little geophysical marvel known as Barton Creek. It is one of the very few crystal-clear streams running through the heart of a large American city and—when it rains hard enough—home to the best whitewater in Texas. The right conditions never last more than a few days. But as with a powder day or a massive ocean swell, the tiny, ephemeral window of opportunity is the whole fun of it. Here is the way it goes: A violent thunderstorm unleashes a roiling wall of water through the canyons. I call a set of friends who have been running this stretch of Barton Creek with me for more than a decade, and we start plotting. The water soon recedes, and then thousands of springs, newly recharged, take over. Soon the creek settles and clears, swollen and rolling now at 300 to 500 cubic feet per second. Then I call in sick. We put in just upstream from my house, at the start of a rollicking seven-mile run through several dozen sets of whitewater rapids, mostly in the Class II–III range. There is everything a kayaker could want: swirling holes, standing waves, ledges, undercut boulders, surfing waves, lovely crystalline water, and almost no sign of urban civilization as you run through sculpted canyons along banks dense with cottonwood, pecan, willow, live oak, and sycamore. After a three-hour paddle, we end in the heart of the city, and a good thing, too—most of the whitewater will be gone in a few days, and it is time to get back to work.